A conventional load drive circuit which has heretofore been proposed (refer to U.S. Pat. No. 4,661,880 in the name of one of the applicants of the present invention) involves an electromagnetic relay contact, and a switch circuit using for example a semiconductor switch element, connected in series in a load feeder circuit. With this load drive circuit, power supply to the load is controlled by inputting a load drive signal to the switch circuit to switch the semiconductor switch element on and off, and if a conduction fault occurs in the semiconductor switch element, this is detected and the electromagnetic relay contacts switched off to forcibly interrupt the load current.
The structural theory of such a conventional load drive circuit is explained with reference to FIG. 9.
In FIG. 9, relay contacts 5A of an electromagnetic relay 5, and a switch circuit 4 using a semiconductor switch element such as an SSR (solid state relay), are connected in series in a load feeder circuit 3 wherein a load 2 is connected in series to a power source 1. The presence or absence of a load current I.sub.L in the load feeder circuit 3 is detected by a zero current detection sensor 7 through a current transformer 6. An output signal I.sub.P of the zero current detection sensor 7 and a load drive signal I.sub.N are input to an OR gate 8, and the contacts 5A of the electromagnetic relay 5 are driven open or closed depending on a logical sum output of the OR gate 8. The load drive signal I.sub.N is made a binary signal (logic value "1", "0") which does not err to logic value "1" at the time of a fault.
The zero current detection sensor 7, is one which gives an output signal I.sub.P of logic value "1" corresponding to a high energy condition, when a load current I.sub.L does not flow in the load feeder circuit, and gives an output signal I.sub.P of logic value "0" corresponding to a low energy condition, when a load current flows. For example the sensor 7 is constructed with a saturable magnetic body core. More specifically, a saturable magnetic body core which is respectively wound with an output lead of the current transformer 6, a primary winding for supplying an alternating current signal, and a secondary winding for receiving the signal. With this construction, when the load current I.sub.L flows in the load feeder circuit 3 of the load 2 so that a current flows in the output lead of the current transformer 6, the saturable magnetic body core becomes saturated. An alternating current signal supplied to the primary winding will therefore not be transmitted to the secondary winding side, and the output signal I.sub.P will not be produced (corresponding to logic value "0"). On the other hand, when the load current I.sub.L does not flow in the load feeder circuit 3 of the load 2 so that a current does not flow in the output lead of the current transformer 6, the saturable magnetic body core does not become saturated. An alternating current signal of the primary winding will therefore be transmitted to the secondary winding side, and the output signal I.sub.P produced (corresponding to logic value "1")
That is to say, the construction of the conventional load drive circuit is such that when a current does not flow to the load 2, or when the switch circuit 4 is switched on under normal conditions with input of the load drive signal I.sub.N, (i.e. when IP=1, or I.sub.N= 1), the electromagnetic relay 5 is excited so that the relay contacts 5A close.
For the excitation condition of the electromagnetic relay 5 of FIG. 9, the closed ("on") and open ("off") of the relay contacts 5A is represented by a binary signal "1" and "0" respectively, with the relay contacts 5A operating under the following conditions with respect to the load drive signal I.sub.N and the load current I.sub.L.
r=I.sub.N inversion I.sub.L
where symbol represents the logical sum, and I.sub.N , I.sub.L and r represent the following situation: ##EQU1##
With the binary signal described hereunder, logic value "1" indicates the presence of voltage or the presence of current, while logic value "0" is for when there is no voltage or no current.
With the above described conventional load drive circuit construction the following two problem points arise.
(1) When the load drive signal I.sub.N is not input (that is I.sub.N =0), if an "on" (conduction) fault occurs in the switch element of the switch circuit 4, then a phenomena commonly known as "hunting" occurs.
This "hunting" is a phenomena wherein the relay contacts 5A repeatedly switch between "on" and "off" in the following manner. ##STR1##
(2) If an "on" fault occurs in the semiconductor switch element, the relay contacts 5A of the electromagnetic relay 5 switch between "on" and "off" in correspondence with the load drive signal I.sub.N.
More specifically, with the construction of FIG. 9, when the load drive signal I.sub.N is not input, since the output signal I.sub.P of the zero current detection sensor 7 becomes IP=0 with the occurrence of an "on" fault in the semiconductor switch element, the relay contacts 5A open. However, when the load drive signal I.sub.N is input, since the output of the OR gate 8 becomes a logic value "1 ", the electromagnetic relay 5 becomes excited causing the relay contacts 5A to close. The relay contacts 5A of the electromagnetic relay 5 thus switch the load current I.sub.L in correspondence with the load drive signal I.sub.N, instead of the semiconductor switch element.
The present invention takes into consideration the above situation, with a primary object of preventing the undesirable situation of the relay contact "hunting" when a conduction ("on") fault occurs in the switch element of the switch circuit. A secondary object is to prevent the undesirable situation of the relay contacts switching between "on" and "off" in correspondence with the load drive signal when a conduction ("on") fault occurs in the switch element of the switch circuit.